1. Field of the Invention
The present invention relates to a piezoelectric ceramic composition for use in a piezoelectric transformer and the like. Particularly, the present invention relates to a piezoelectric ceramic composition in which the mechanical-quality factor Qm and the electromechanical coupling factor Kp are superior, and to a high power output piezoelectric transformer made of the same composition.
2. Description of the Prior Art
The discovery of the piezoelectric ceramic was such that BaTiO3 was discovered at the middle of 1940s, and then, Pb(Zr,Ti)O3(to be called simply xe2x80x9cPZTxe2x80x9d below) having more superior piezoelectric properties was developed thereafter. This material has been widely applied to high voltage generators, ultrasonic apparatuses, sound apparatuses, communication apparatuses, and various other sensors.
PZT is a solid solution form of PbZrO3 and PbTiO3, and has a perovskite structure, while it has superior piezoelectric properties. In order to make the composition variation easier than this two-component system and to improve the piezoelectric properties, there have been developed three-component system composite perovskite compounds. Among these three-component system compounds, attention has been focused on: Pb(Mg,Nb)O3xe2x80x94Pb(Zr,Ti)O3, Pb(Mg,Ta)O3xe2x80x94Pb(Zr,Ti)O3, and Pb(Mn,Nb)O3xe2x80x94Pb(Zr,Ti)O3.
Recently there has been reported a piezoelectric ceramic composition in which the coercive electric field and the electromechanical factor are high, and the grain size is made fine by a composition system of Pb0.94Ba0.06)(Mn1/3Nb2/3)0.075(Zr0.52Ti0.48)0.925O3+0-2 mol % Ceo2 (Dielectric constant=799, Qm=1, 285, Kp=0.54, Ec=10.7 KV/cm, Tc=332xc2x0 C., grain size=2.57 xcexcm, J. H. Yoo; Journal of Korean Electric Academy, 48C, No. 9, P811, 1999). These three-component system piezoelectric ceramic compositions are low in the dielectric constant and in the mechanical quality factor, and therefore, the use of them is limited to the low current output apparatuses such as the inverter of the cold cathode ray tube for illuminating the rear face of the LCD display. Their power is in the order of 2-3 watts, and therefore, they cannot be used in the general fluorescent lamps which consume a considerable amount of electric current.
The impedance of the general fluorescent lamps is as low as several hundred xcexa9 to several xcexa9 when lighted, whereas their output power is high i.e., in the order of 10-100 watts. If the piezoelectric ceramic material is to be used for a high power, first the generation of heat, the non-linearity, the degradation of the piezoelectric properties and the mechanical strength have to be solved. In order to achieve this, first the mechanical quality factor and the electromechanical coupling factor have to be high even under a high input power, so that the energy conversion efficiency can be improved to decrease the internal loss so as to decrease the thermal radiation. Second, the mechanical oscillations are high, and therefore, the grain size has to be made fine, thereby increasing the mechanical strength.
If the piezoelectric material is to be used in the transformer for a high power. fluorescent lamp, then the structure of the transformer as well as the physical properties of the material is important. FIG. 1 illustrates a typical example of a piezoelectric transformer which is used in an inverter. This transformer is the Rosen type in which the thickness oscillations and the lengthwise oscillations are utilized. In the transformer 10 of FIG. 1, a pair of input electrodes 14 are respectively formed on the bottom and top of a piezoelectric block 12, and the input electrodes 14 are polarized in the thickness direction. An output part consists of an electrode 16 which is disposed on a side of the piezoelectric block, and is polarized in the lengthwise direction. If the piezoelectric transformer is to be stepped up, then an ac voltage corresponding to the resonance frequency is supplied to the input electrodes 14. Then the supplied electrical signals are converted to strong mechanical oscillations in the thickness direction near the input electrodes of the piezoelectric block 12. These oscillations lead to lengthwise oscillations of the output side, and consequently, a stepped-up high voltage with a frequency same as the input frequency is outputted through the output electrode 16. The stepping-up of the voltage becomes maximum when the frequency of the input voltage is same as the oscillation frequency of the output side. Under this condition, the ratio of the stepping-up of the piezoelectric transformer depends on the impedance of the load. That is, if a low impedance of load is connected to the output side, the ratio of the stepping-up becomes less than several scores. The magnitude of the load impedance is different depending on the kind of lamps in the case where the piezoelectric transformer is used in the cold cathode ray tube or in the fluorescent lamp. However, if the piezoelectric transformer is manufactured at the optimum conditions, then a high stepping-up ratio can be maintained. Thus in the normal case where a high impedance is connected before the lighting, and where the load impedance is lowered after the lighting, a sufficient stepping-up ratio can be maintained, so that it can be used in the cold cathode ray tube or in the fluorescent lamp.
Recently, there has been known a filter 10 in which the oscillation mode is as shown in FIG. 2. An input electrode 14 is formed on the top of a piezoelectric block 12, and an output electrode 16 is formed around the input electrode 14 keeping a certain distance from the latter. On the bottom of the piezoelectric block 12, there is formed a second electrode 18 which is a common electrode. If a voltage is supplied into the input electrode 14, then the supplied electrical signals are converted to mechanical oscillations from the center toward the peripheral portions, and then, output signals proportional to the mechanical oscillations are outputted through the output electrode 16. However, if this structure is used in a high power apparatus, then stress is imposed on the middle of the edge portions, with the result that the device is damaged or its efficiency is degraded.
As described above, if the piezoelectric ceramic is to be used in a high output power apparatus, then the problems of the mechanical quality factor Qm and the electromechanical coupling factor Kp have to be solved first, and a proper structure of the transformer has to be provided also.
The present invention is intended to solve the above described problems of the conventional techniques.
Therefore it is an object of the present invention to provide a piezoelectric ceramic composition in which the dielectric constant, the electromechanical coupling factor and the mechanical quality factor are superior, and a high output power piezoelectric transformer obtained by using the piezoelectric ceramic composition.
It is another object of the present invention to provide a high output power piezoelectric transformer, in which the piezoelectric ceramic composition is used to construct the transformer so that a high output power characteristics are satisfied, and the operation is stable, by designing particular electrodes.
In achieving the above objects, the piezoelectric ceramic composition according to the present invention includes: Pb1xe2x88x92aSra[(Ni1/2W1/2)b(Mn1/3Nb2/3)c(Zr1xe2x88x92xTix)1xe2x88x92bxe2x88x92c]O3+kPbO, where a is 0-0.06 mol %, b is 0.01-0.05 mol %, c is 0.01-0.09 mol %, x is 0.47-0.53 mol %, and k is 0.1-0.7 wt %.
In another aspect of the present invention, the high output power piezoelectric transformer according to the present invention obtained by using a piezoelectric ceramic composition satisfying Pb1xe2x88x92aSra[(Ni1/2W1/2)b(Mn1/3Nb2/3)c(Zr1xe2x88x92xTix)1xe2x88x92bxe2x88x92c]O3+kPbO, where a is 0-0.06 mol %, b is 0.01-0.05 mol %, c is 0.01-0.09 mol %, x is 0.47-0.53 mol %, and k is 0.1-0.7 wt %.
In still another aspect of the present invention, the high output power piezoelectric transformer according to the present invention includes: a piezoelectric block composed of Pb1xe2x88x92aSra[(Ni1/2W1/2)b(Mn1/3Nb2/3)c(Zr1xe2x88x92xTix)1xe2x88x92bxe2x88x92c]O3+kPbO, (where a is 0-0.06 mol %, b is 0.01-0.05 mol %, c is 0.01-0.09 mol %, x is 0.47-0.53 mol %, and k is 0.1-0.7 wt %.); a first electrode consisting of an inner electrode and an outer electrode, the inner electrode being formed at a center of a top of the piezoelectric block, and the outer electrode being formed around the inner electrode by being separated by an isolating region; and a second electrode formed on the bottom of the piezoelectric block.